Debian Stable or Debian Testing: Which Linux is right for you?

How does the Debian GNU/Linux Testing distribution compare to their Stable distribution, and to some of the Debian derivatives that I have recently looked at?

From ZDNet:

I have written several times recently about Debian GNU/Linux and several distributions derived from it. As a few people have pointed out, though, when I wrote about Debian itself I only covered the "Stable" distribution (currently 'Wheezy'), whereas the other distributions are almost always derived from the Debian "Testing" distribution (currently 'Jessie').

This leaves open some valid questions about how Debian Jessie itself stacks up against those other distributions - and even the more basic question of how does Jessie compare with Wheezy? Since the first alpha release of Jessie was just made a week or so ago, this seems like a good time to take a look at it in detail.